1. Field of Invention
This invention relates to digital communication systems, and more particularly to a method and system for increasing the throughput of information bits transmitted on the same channel or frequency in a Multiuser Detection (“MUD”) based multiple access communications system by increasing the number of dimensions of each transmitted signal thereby supplying the receivers with additional data in which to decode and detect the incoming transmitted signals.
2. Description of Related Art
Multiple access communication systems allow the transmission of multiple digital data streams between multiple transmitting and receiving devices. Cellular or Personal Communication Systems (PCS) systems provide high quality voice service in a wide-ranging geographical coverage area at relatively low cost. However, since many users transmit energy on the same communications channel, a number of inherent difficulties arise, particularly when a large number of user receivers attempt to detect the information associated with a particular user when there is heavy signal interference created by other users of the system at the same time. Typically the signal of interest cannot be received or the quality of reception is significantly degraded.
One way of solving this problem is to separate the interfering transmissions at the receiver, using signal processing techniques. However, today's current state of the art receivers are not capable of detecting and decoding the information associated with each user under conditions of heavy interference. Another solution to the co-channel interference problem is to decrease the number of users per channel. This, of course, is not an attractive option for telecommunication companies, since obtaining the maximum number of users or managing peak volume transmission periods are important business objectives.
Several techniques have become available in recent years to improve results in co-channel multiple access communications systems. Frequency-Division Multiple Access (FDMA) assigns a different frequency to each user. In a cellular telephone configuration this poses problems because all proximate cells must operate on different frequencies. However, frequency bands may be re-used, provided that the same frequency cells are positioned at a certain distance apart. A further drawback with FDMA schemes is that users will pay full-time for their assigned frequency regardless of their actual use of the system. Additionally, the number of users will be restricted to the number of dedicated frequencies that can be provided and the base station must have one dedicated modem for each user, therefore leading to high hardware and operational costs.
In Time Division Multiple Access (TDMA) technology, multiple channels of data are temporally interleaved, i.e. each signal is assigned to a different time interval and the signals are transmitted individually, according to their assigned time slot. However, in a TDMA system, all transmitters and receivers must have access to a common clock, as time-synchronization among the users is required.
Code Division Multiple Access (CDMA) is another multiplexing technique wherein for each communication channel, the signals are encoded using a sequence known to the transmitter and the receiver for that specific channel. In CDMA, all users use the same frequency at the same time. However, before transmission, the signal from each user is multiplied by a distinct signature waveform. The signature waveform is a signal that has a larger bandwidth than the information-bearing signal from the user. However, in a CDMA system, the total level of co-channel interference limits the number of active users at any instant of time.
Multiuser detection (“MUD”) systems take full advantage of all information available at the receiver, by making use of any “knowledge” that the receiver has about the interfering signals. Because the number of users that can be packed into a MUD-based multiple access (MA) system is a function of the number of independent dimensions over which the set of signals is spread (the dimension of the span of the set of signals), the total number of users in the system can be increased if more dimensions are used for transmitting the signals and the same dimensions are accessible at the receiver.
Further, if the received signals are each defined by a polarization ellipse that is unlike the polarization ellipses for the other signals within a given frequency channel, the receiver would have additional data in which to distinguish between similar signals.
In addition to expanding the number of dimensions, favorably “spreading” the received signals out over those dimensions can also allow for increases in the number of users a MUD-based system can accommodate. For example, typical signaling sets for multiuser communications do not include as a free parameter the reference amplitude of each user. In the IS95 code division protocol, amplitude is controlled completely for purposes of power control to meet a signal-to-noise specification (all users ideally being received with the same signal-to-noise ratio (SNR)). By not including amplitude as a potential design choice, the extra advantage offered to the MUD is not exploited. The aggregate throughput of a multiple access system is limited if amplitude is not exploited. Fewer users are supported by MUD-based systems not explicitly controlling user amplitude.
Accordingly, what is needed in the art is a multiuser detection system that supplies one or more additional dimensions such as, for example, a randomly selected polarization ellipse for each signal and an easily achievable, but advantageous, assignment of the value of any variable parameter such as the amplitude (power) of the received signals. In this fashion, because there are an increased number of dimensions or parameters used for assigning the transmitted signals, and the same dimensions or parameters are measured at the receiver, multiple signals transmitted within the same frequency band can be detected and separated thereby allowing for an increase in the total number of users in a given MA system.
It is, therefore, to the effective resolution of the aforementioned problems and shortcomings of the prior art that the present invention is directed.